(A) Field of the Invention
The present invention relates to an uninterruptible power system (UPS) capable of supplying a stable power to a load even when an AC power failure occurs.
(B) Description of Related Art
When a power failure occurs, damages such as data loss are feared in a so-called information processing apparatus such as a server, a router and a storage. As countermeasures for power failure, an uninterruptible power system (UPS) has been installed which uses an energy accumulation unit such as a rechargeable battery. UPS includes an AC output type and a DC output type. The former is more popular and inserts an uninterruptible power system between a commercial AC power source and a subject apparatus. Basing upon the DC output characteristics, the latter generally adopts the structure that an uninterruptible power system is connected to a DC line at the succeeding stage of an AC/DC converter of an apparatus. Examples of the latter are disclosed in Patent Document 1 and Japanese Patent Laid-open Publication No. 2002-171692.
According to these conventional technologies, an AC power is supplied from an AC power source to a rectification circuit and to a power factor correction circuit. A DC power is then supplied to a DC output DC/DC converter whereat the DC power is converted into a plurality of DC powers having desired voltages and currents which are supplied to loads. In a power failure state, a DC power is supplied from a battery to a backup converter and to the DC output DC/DC converter. In a power normal state, the battery is charged with a charger.
Patent Document 1:
JP-A-2000-116029
(Abstract and Others)
In a conventional backup power source, during a power failure, a DC voltage, e.g., 380 V, is required to be generated from a battery via a backup converter. It is conceivable that a lead battery is often used as the backup battery. The voltage of a lead battery per cell is about 12 V. From recent requirements for compact and light weight batteries, it is supposed that rechargeable batteries having a higher energy density are used, including nickel metal hydride (NiMH) batteries, lithium-ion (Li-ion) batteries and the like. The NiMH battery is about 1.2 V per cell, and the Li-ion battery is about 3.7 V per cell. It is also expected to use a high capacity charge accumulation unit such as an electric double layer capacitor and a fuel battery expected to be in practical use in the future.
As a backup converter, a non-isolated, boost chopper type converter may be used which has a relatively simple and compact structure and is easy to be controlled. A ratio of a low side voltage to a high side voltage of this converter is required to be suppressed to a range from about 1.2 times to about 3 times when the circuit efficiency is taken into consideration. The number of battery cells is therefore about 105 to 264 cells for an NiMH battery and about 34 to 86 cells for a Li-ion battery.
Such a large number of serially connected battery cells poses some problems of a variation in States of Charges (SOC) of cells and the reliability against occurrence of a cell shortage failure. From this reason, system sides require that the number of serially connected cells is to be suppressed to 30 cells or less at the most.
In order to reduce the number of serially connected cells, it is essential to realize a high boost ratio of an isolated converter using a transformer. However, this results in not only a complicated structure but also a high cost and a large size.